1. Field of the Invention
The present invention relates to a semiconductor device. In this specification, a semiconductor device refers to a semiconductor element itself or a device including a semiconductor element. Examples of such a semiconductor element include, for example, a thin film transistor. Accordingly, a semiconductor device also includes a device such as a liquid crystal display device in its category.
2. Description of the Related Art
As one kind of a volatile memory, a static random access memory (SRAM) is widely known.
Meanwhile, in recent years, a metal oxide having semiconductor characteristics (hereinafter referred to as an oxide semiconductor) has attracted attention. An oxide semiconductor can be applied to a transistor (see Patent Documents 1 and 2).